Functions of nitric oxide (NO) in the cardiovascular system, respiratory system, digestive system, urinary system and nervous system have been elucidated in the fields of biochemistry and physiology, and thus research on NO has been increasingly conducted. The vasodilation and anti-thrombosis characteristics of NO (Radomski, M. W et al., Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 5193-5197) have been applied to development of drugs in the cardiovascular system, and the phagocytosis of NO with which macrophages are associated has been applied to development of a tumoricidal agent (Langrehr, J. M et al., Transplantation 1993, 55, 1205-1212), an antibiotic and a bactericidal agent. Also, NO is an intercellular signaling molecule (Ohta, A. et al Neurosci. Lett. 1993, 158, 33-35). In order to investigate a specific reaction mechanism of NO, it is very important to directly measure NO in the human body or in the inside or outside of cells producing NO.
Meanwhile, researchers have started to display interest in storage or delivery of NO in the human body as physiological and medical roles of NO become important. In particular, researchers have taken a great interest in that NO, which is a radical molecule having high reactivity, shows stable physiological activities due to its short life span (Cha, W. S. et al., Biomaterials 2007, 28(1), 19-27). RSNO is a solution to such a problem. That is, NO is modified into RSNO in cells for its delivery (Zhang, Y. et al., Proc. Natl. Acad. Sci. USA, 2004, 101, 7891-7896).
In recent years, much research has been conducted to measure a concentration of RSNO associated with the storage and delivery of NO.
RSNO is represented by the following Formula 1, and is formed by nitrosylating a sulfhydryl group (R-SH) with NO. Also, physical/chemical properties of RSNO decomposed by color or light are different according to functional groups R, and may play various roles or be present in various positions when RSNO is present as an in vivo molecule.

FIG. 1 shows representative RSNO compounds present in a living body (Williams D. L-H. et al., Acc. Chem. Res. 1999, 32, 869-876). Unlike conventional RSNO compounds which are unstable in the air, there are a limited number of RSNO compounds which are stable in the air.
The biochemical activities of RSNO are realized by generation of NO caused by the breakdown of an S—N bond. FIG. 2 shows a mechanism in which NO is produced from RSNO. FIGS. 2A and 2B show decomposition caused by a metal catalyst, Cu ((I) and (II)), FIG. 2C shows photo-induced decomposition caused by light, and FIG. 2D show decomposition caused by cross nitrosation.
RSNO is present at a low concentration from several nM to several μM in a living body, and present at different amounts according to body regions. Therefore, a sensor has to show very high sensitivity to measure RSNO. Also, since RSNO tends to be unstably and easily decomposed in the air, a sensor should meet requirements such as being able to desirably measure RSNO in a living body and having enhanced selectivity to RSNO among numerous nuisance species present in the living body.
A conventional method of measuring RSNO includes a method using an electrochemical NO sensor. That is, RSNO is indirectly quantified by decomposing RNSO using Cu ((I) and (II)) or a biochemical catalyst as the metal catalyst and measuring generated NO using an electrochemical NO sensor. However, such a method has a problem in that, when RSNO and NO are present together in a sample, the sensor does not differentiate the existing NO from NO generated by decomposition of RSNO using the catalyst. In other words, during measurement of RSNO, NO acts as a nuisance species. Therefore, it is impossible to measure a precise concentration of RSNO in a living body using the above-described method.
In order to solve such a problem, an attempt has been made to constitute a differential electrode system (Cha, W. et al., Biosens. Bioelectron. 2009, 24, 2441-2446) for correction. That is, this is a method of measuring a concentration of RSNO using two working electrodes, which includes measuring concentrations RSNO and NO together using one working electrode, measuring only a concentration of NO using the other working electrode and calculating a difference value between sensor signals obtained from the two working electrodes. However, such a method has problems in that the sensitivity to NO should be precisely the same in the two working electrodes, and a measurement device is complicated and larger in scale due to the use of the two working electrodes, which makes it difficult to perform direct measurements in the living body.